Certain plants excrete sweet juice, apparently for the sake of eliminating something injurious from the sap: this is effected, for instance, by glands at the base of the stipules in some Leguminosae, and at the backs of the leaves of the common laurel.

This juice, though small in quantity, is greedily sought by insects; but their visits do not in any way benefit the plant.

Now, let us suppose that the juice or nectar was excreted from the inside of the flowers of a certain number of plants of any species.

Insects in seeking the nectar would get dusted with pollen, and would often transport it from one flower to another.